Friction, grit, or grid drive systems for moving strips or webs of sheet material longitudinally back and forth along a feed path through a plotting, printing, or cutting device are well known in the art. In such drive systems, friction (or grit or grid) wheels are placed on one side of the strip of sheet material (generally vinyl or paper) and pinch rollers, of rubber or other flexible material, are placed on the other side of the strip, with spring pressure urging the pinch rollers and material against the friction wheels. During plotting, printing, or cutting, the strip material is driven back and forth, in the longitudinal or X-direction, by the friction wheels while, at the same time, a pen, printing head, or cutting blade is driven over the strip material in the lateral or Y-direction.
These systems have gained substantial favor due to their ability to accept plain (unperforated) strips of material in differing widths. However, the existing friction feed systems experience several problems. One problem is longitudinal slippage or creep error in the X-direction. The longitudinal slippage or creep occurs when the strip material moves either too slowly or too fast, respectively, in the X-direction. This problem is most pronounced in long plots, i.e. those two or more feet in length, and those in which the strip material moves back and forth in the X-direction with respect to a tool head such as a plotting pen, print head, or cutting blade. Longitudinal slippage or creep is highly undesirable because the operations performed on the strip material become inaccurate.
Another error that occurs in friction feed systems is a skew error. The skew error will arise as a result of strip material being driven unevenly between its two longitudinal edges, causing the strip material to assume a cocked position. The error is integrated in the lateral or Y-direction and produces an increasing lateral position error as the strip material moves along the X-direction. The error is often visible when the start of one object must align with the end of a previously plotted object. In the worst case, such lateral errors result in the strip drifting completely off the friction wheel.